Method of making polymer electrolyte electrochemical cells

Abstract

The invention relates to a method for the preparation of a polymer electrolyte electrochemical cell using an electrolyte precursor, said precursor comprising one or more solvents, one or more salts and a polymer which dissolves in the solvent at a first temperature (T dissol ) and which is capable of forming a gel on subsequent cooling following heating to a second temperature (T gel ). T dissol being lower than T gel , which method comprises: (a) heating the electrolyte precursor to T dissol ; (b) optionally cooling the electrolyte precursor, (c) incorporating the electrolyte precursor into the electrochemical cell; (d) heating the electrochemical cell to T gel ; (e) cooling the polymer electrochemical cell to ambient temperature to bring about gelling of the polymer electrolyte. Preferably the polymer is a homopolymer or copolymer from the group of monomers of vinyl fluoride, vinylidenefluoride, trifluoroethylene, tetrafluoroethylene and hexafluoropropylene.

1. A method for the preparation of a polymer electrolyte electrochemical cell using an electrolyte precursor, said precursor comprising one or more solvents, one or more salts and a polymer which dissolves in the solvent at a first temperature, T dissol , and which is capable of forming a gel on subsequent cooling following heating to a second temperature, T gel , wherein T dissol  is lower than T gel , which method comprises:
 heating the electrolyte precursor to T dissol ;  
 (a) optionally cooling the electrolyte precursor;  
 (b) incorporating the electrolyte precursor into the electrochemical cell;  
 (c) heating the electrochemical cell to T gel ; and  
 (d) cooling the polymer electrochemical cell to ambient temperature to bring about gelling of the polymer electrolyte.  
 
     
     
       2. The method according to  claim 1 , in which said polymer is a homopolymer or copolymer selected from the group of monomers consisting of vinyl fluoride, vinylidenefluoride, trifluoroethylene, tetrafluoroethylene and hexafluoropropylene. 
     
     
       3. The method according to  claim 1 , in which said polymer is a copolymer of vinylidenefluoride and hexafluoropropylene. 
     
     
       4. The method according to  claim 3 , in which said copolymer of vinylidenefluoride and hexafluoropropylene has a molecular weight in the range 50,000-500,000, and a weight ratio of vinylidenefluoride and hexafluoropropylene in the range 80:20 to 90:10. 
     
     
       5. The method according to  claim 1 , in which the electrolyte comprises one or more solvent(s) selected from the group consisting of:
 (a) alicyclic carbonates represented by the following general formula: 
   —C(═O)═O—CR 1 R 2 —[CR 3 R 4 ] m —CR 5 R 6 —O—,  
 
 
       wherein each of R 1 , R 2 , R 3 , R 4 , R 5  and R 6  independently represents hydrogen or a C 1 -C 4  alkyl group and m is 0 or 1;
 (b) aliphatic carbonates represented by the general formula R 7 [OC(O)] p OR 8 , wherein each of R 7  and R 8  independently represents a C 1 -C 4  alkyl group, and p is an integer equal to 1 or 2;  
 (c) lactones in the form of cyclic esters represented by the general formula: 
   —C(═O)—CR 9 R 10 —CR 11 R 12 —[CR 15 R 16 ] r —CR 13 R 14 —O— 
 
 
       wherein each of R 9 , R 10 , R 11 , R 12 , R 13 , R 14 , R 15  and R 16  independently represents hydrogen or a C 1-2  alkyl group and r is 0 or 1;
 (d) esters represented by the formula R 17 [C(O)]OR 18 ] t , wherein each of R 17 , R 18  and R 19  independently represents hydrogen or a C 1 -C 2  alkyl group, and t is 0 or an integer equal to 1 or 2; and  
 (e) glymes represented by the general formula R 20 O(R 21 O) n R 22 , in which each of R 20  and R 22  independently represents a C 1-2  alkyl group, R 21  is —(CR 23 R 24 CR 25 R 26 )— wherein each of R 23 , R 24 , R 25  and R 26  independently represents hydrogen or a C 1 -C 4  alkyl groups, and n is an integer from 2 to 6.  
 
     
     
       6. The method according to  claim 1 , in which the electrolyte comprises one or more salts selected from the group consisting of alkali metal or ammonium salts of ClO 4 , CF 3 SO 3 , AsF 6 , PF 6  and BF 4 . 
     
     
       7. The method according to  claim 1 , in which the electrolyte comprises solvent(s), salt(s) and polymer in the compositional range from 63:25:12 to 94:5:1 percent of the total weight of the electrolyte system. 
     
     
       8. The method according to  claim 1  in which the electrolyte is confined in a separator consisting of a porous structure made of a polymer. 
     
     
       9. The method according to  claim 8 , in which the separator has a woven or non-woven structure having a pore size in the range of 10×10 nm to 1×1 mm. 
     
     
       10. The method according to  claim 8 , in which the separator has a thickness of 10-100 μm. 
     
     
       11. The method according to  claim 1 , in which the electrochemical cell has a negative electrode structure comprising one or more compounds selected from the group consisting of graphite, coke, mesocarbon microbeads, carbon black, aluminum, silicon and tin, and a positive electrode structure comprising one or more compounds selected from the group consisting of lithium manganese oxides, lithium cobalt oxides and lithium nickel oxides. 
     
     
       12. The method according to  claim 1 , in which the dissolution temperature T dissol  is in the range 45-80° C. and the gelling temperature T gel  is in the range 75-100° C., with the proviso that T gel  should be higher than T dissol . 
     
     
       13. The method according to  claim 1 , in which the electrochemical cell is wound prior to electrolyte impregnation. 
     
     
       14. The method according to  claim 1 , in which the electrolyte incorporation is carried out by pouring or injection. 
     
     
       15. The method according to  claim 1 , in which the electrodes display such a porosity which is such as to allow diffusion of a relatively solvent-rich electrolyte phase into the pores of the electrodes, leaving a relatively polymer-rich electrolyte phase in the volume between the electrodes. 
     
     
       16. The method according to  claim 1  wherein the gap between electrode laminates of the electrochemical cell is smaller than at least the largest polymer particles so that the electrolyte in the vicinity of the edge of the laminate and outside the laminate contains a larger amount of polymer than the electrolyte between the electrodes. 
     
     
       17. The method according to  claim 4 , wherein the copolymer of vinylidenefluoride and hexafluoropropylene has a molecular weight in the range of 100,000-300,000. 
     
     
       18. The method according to  claim 5 , wherein the electrolyte comprises one or more solvents selected from the group consisting of
 (a) alicylic carbonates selected from the group consisting of ethylene carbonate and propylene carbonate;  
 (b) aliphatic carbonates selected from the group consisting of dimethyl carbonate and diethyl carbonate;  
 (c) lactones selected from the group consisting of γ-valerolactone and γ-butyrolactone;  
 (d) esters selected from the group consisting of (2-methoxyethylacetate) and ethylacetate; and  
 (e) glymes wherein R 20  and R 22  are methyl and R 23 , R 24 , R 25  and R 26  are hydrogen.  
 
     
     
       19. The method according to  claim 7 , wherein the electrolyte comprises solvent(s), salt(s) and polymer in the compositional range 75:17:8 to 88:8:4 percent of the total weight of the electrolyte system. 
     
     
       20. The method according to  claim 8 , wherein the separator consists of a porous structure made of a polymer selected from the group consisting of polyethylene, polypropylene, polycarbonate, and cellulose. 
     
     
       21. The method according to  claim 10 , wherein the thickness is 10-25 μm. 
     
     
       22. The method according to  claim 11 , wherein
 the negative electrode structure comprises one or more compounds selected from the group consisting of graphite and mesocarbon microbeads; and  
 the positive electrode structure comprises LiMn 2 O 4  of spinel structure.  
 
     
     
       23. The method according to  claim 12 , wherein T dissol  is in the range 65-75° C., and T gel  is in the range of 80-90° C. 
     
     
       24. The method according to  claim 6 , wherein the electrolyte comprises one or more salts selected from the group consisting of LiPF 6  and LiBF 4 . 
     
     
       25. An electrochemical cell having (i) a negative electrode structure comprising one or more compounds selected from the group consisting of graphite, coke and mesocarbon microbeads, (ii) a positive electrode structure comprising lithium manganese oxide, and (iii) a gelled polymer electrolyte, 1-12% by weight of the total weight of the electrolyte being said polymer, wherein said polymer is a homopolymer or copolymer selected from the group of monomers consisting of vinyl fluoride, vinylidenefluoride, trifluoroethylene, tetrafluoroethylene and hexafluoropropylene, said electrochemical cell being obtained by or being obtainable by a method comprising:
 (a) heating an electrolyte precursor, comprising one or more solvents, one or more salts and a polymer which dissolves in the solvent at a first temperature, T dissol , and which is capable of forming a gel on subsequent cooling following heating to a second temperature, T gel , to the temperature T dissol , wherein T dissol  is lower than T gel ;  
 b) cooling the electrolyte precursor  
 (c) incorporating the electrolyte precursor into the electrochemical cell;  
 (d) heating the cell to T gel ; and  
 (e) cooling the polymer electrochemical cell to ambient temperature to bring about gelling of the polymer electrolyte.  
 
     
     
       26. An electrochemical cell having (i) a negative electrode structure comprising one or more compounds selected from the group consisting of graphite, coke, and mesocarbon microbeads, (ii) a positive electrode structure comprising one or more compounds selected form the group consisting of lithium manganese oxides, lithium cobalt oxides, and lithium nickel oxides, and (iii) a gelled polymer electrolyte, 1-12% by weight of the total weight of the electrolyte being said polymer, wherein said polymer is a copolymer of vinylidenefluoride and hexafluoropropylene, said electrochemical cell being obtained by or being obtainable by a method comprising:
 (a) heating an electrolyte precursor, comprising one or more solvents, one or more salts and a polymer which dissolves in the solvent at a first temperature, T dissol , and which is capable of forming a gel on subsequent cooling following heating to a second temperature, T gel , to the temperature T dissol , wherein T dissol  is lower than T gel ;  
 (b) cooling the electrolyte precursor  
 (c) incorporating the electrolyte precursor into the electrochemical cell;  
 (d) heating the cell to T gel ; and  
 (e) cooling the polymer electrochemical cell to ambient temperature to bring about gelling of the polymer electrolyte.  
 
     
     
       27. An electrochemical cell having (i) a negative electrode structure comprising one or more compounds selected from the group consisting of graphite, coke, and mesocarbon microbeads, (ii) a positive electrode structure comprising one or more compounds selected form the group consisting of lithium manganese oxides, lithium cobalt oxides, and lithium nickel oxides, and (iii) a gelled polymer electrolyte, 4-8% by weight of the total weight of the electrolyte being said polymer, wherein said polymer is a homopolymer or copolymer selected from the group of monomers consisting of vinyl fluoride, vinylidenefluoride, trifluoroethylene, tetrafluoroethylene and hexafluoropropylene, said electrochemical cell being obtained by or being obtainable by a method comprising:
 (a) heating an electrolyte precursor, comprising one or more solvents, one or more salts and a polymer which dissolves in the solvent at a first temperature, T dissol , and which is capable of forming a gel on subsequent cooling following heating to a second temperature, T gel , to the temperature T dissol , wherein T dissol  is lower than T gel ;  
 (b) cooling the electrolyte precursor  
 (c) incorporating the electrolyte precursor into the electrochemical cell;  
 (d) heating the cell to T gel ; and  
 (e) cooling the polymer electrochemical cell to ambient temperature to bring about gelling of the polymer electrolyte.